The short answer is: You will need the following documents to enter Costa Rica:
1. A passport that is valid for at least 6 months after your arrival to Costa Rica.
2. An airline ticket in your name to leave Costa Rica within 90 days of your arrival date.
Citizens of the United States, Canada and most European countries are not required to have a visa to visit Costa Rica.
Visit the Costa Rica Embassy website for your specific country.
You will be required to present a yellow fever vaccination certificate if you are traveling from any of the following countries: Angola, Benin, Burkina Faso, Cameroon, Democratic Republic of Congo, Gabon, Gambia, Ghana, Guinea, Liberia, Nigeria, Sierra Leone, Sudan, Bolivia, Brazil, Colombia, Ecuador, Peru, Guyana, and Venezuela.
Once in Costa Rica, you are required to carry your passport or a copy of it which includes the complete photo and data page as well as the visa stamp page.